There has been a great deal of interest in developing a thin display apparatus such as a liquid crystal display apparatus, an organic EL (Electro Luminescence) display apparatus, and an FED (Field Emission Device) display apparatus. Among them, the liquid crystal display apparatus and a thin EL display apparatus have particularly attracted attention as a display apparatus for mobile phones and portable PCs, due to the lightness and low power consumption.
The portable devices have become multifunctional so that the power consumption thereof has become increased. Thus it has been strongly demanded to reduce the power consumption of various means provided in the portable device as well as to increase the capacity of a battery for power supply. Compared to other means in the portable device, display means is generally used at length so as to consume a lot of electricity, thereby it has been demanded to further reduce the power consumption of the display means to elongate the hour of use. Therefore the first objective of the present invention is to further reduce the power consumption of the display means.
Moreover, lightness and portability are quintessence of portable devices so that the display means has been required to be smaller and thinner, along with the reduction of the power consumption. That is to say, the display means includes a drive circuit (drive means or driver), etc. for displaying an image as well as a display section on which images are displayed, and to downsize the portable devices, the proportions of the display section is required to be as large as possible, while the drive circuit, etc. are required to be small and thin as much as possible. This reduction of the size and thickness of the display means is the second objective of the present invention.
Generally a liquid crystal panel (liquid crystal display apparatus) is used as the display means of the portable devices. This liquid crystal panel can achieve both the first and second objectives so as to be widely used as the display means of the portable devices.
Incidentally, there are several types of the liquid crystal panel classified by the difference in driving method and mode of liquid crystal, and among them, a TFT (Thin Film Transistor) drive active matrix TN (Twisted Nematic) liquid crystal panel (hereinafter, will be simply referred to as a TFT liquid crystal panel) has characteristics in high quality displaying and fast drive speed. Thus this liquid crystal panel is highly promising to be used as the display means for the multipurpose portable devices.
However, as the display means of the portable device, a simple matrix drive STN (Super Twisted Nematic) liquid crystal panel (hereinafter, will be simply referred to as a simple STN liquid crystal panel) has generally been adopted. Apart from the relatively high cost, the reason of the poor demand for the TFT liquid crystal panel is mainly considered that the power consumption thereof is too large to be adopted as the display means of the portable device.
Liquid crystal panels basically consume small amounts of electric power compared to conventional CRT display apparatuses. However, as a kind of the liquid crystal panel, the TFT liquid crystal panel can realize high-definition displaying but requires relatively large amounts of electric power, so as to be inadequate for the display means of the portable device.
Thus, there have been various efforts to achieve the above-identified first objective. For instance, a technique (1) disclosed in Japanese Laid-Open Patent Application No. 2000-227608 (Tokukai 2000-227608; published on Aug. 15, 2000) attempts to reduce the power consumption of the TFT liquid crystal panel by providing image memories outside the display screen of the display apparatus.
More specifically, conventional TFT liquid crystal panels are arranged such that the data written in all pixels of the TFT liquid crystal panel are updated in every frame time to realize displaying without flicker, and this increases the power consumption.
In the meantime, the technique (1) adopts the image memories, and hence when a static image is displayed, it is unnecessary to update the image in every frame time. Moreover, the image memories are arranged in a bit map manner so that the image memories and pixels of the display section is assigned to the same address. Therefore, when a part of the displayed image is altered, only image data of a single line including pixels corresponding to the altered part of the image is required to be updated, and hence it is possible to realize a low-power-consumption TFT liquid crystal panel.
Also, there have been various efforts to achieve the above-identified second objective. For instance, according to a technique (2) disclosed in Japanese Laid-Open Patent Application No. 2000-330527 (Tokukai 2000-330527; published on Nov. 30, 2000), when a grayscale display of m bits is carried out, a D/A conversion circuit generates the voltage of n bits (m>n), and the grayscale display of remaining (m−n) bits are conducted in a time ratio grayscale manner.
A digitally driven TFT liquid crystal panel adopts the D/A conversion circuit (D/A conversion means) which converts digital image data, which is supplied from the outside, to analog image data. The performance of multi-grayscale display is an important factor to realize the high-definition displaying, and to improve the performance of the multi-grayscale display, it is necessary to improve the performance of the D/A conversion circuit. However, the improvement above requires to enlarge the D/A conversion circuit so that the space occupied with the circuit is expanded.
Moreover, in the manufacturing process of the TFT liquid crystal panel, the D/A conversion circuit is often manufactured in a polysilicon TFT process along with a TFT, etc. In this case, however, the arrangement of the circuit is so complicated that the drive circuit (especially a source driver) of the TFT liquid crystal panel requires a larger area.
Thus, the technique (2) is arranged such that among digital image data of m bits (m is an integer not less than 2) supplied from the outside, data of n bits (n is an integer not less than 2 and not more than m) is used as information for voltage grayscale, and data of m−n bits is used as information for time ratio grayscale. In this arrangement, the voltage grayscale and the time ratio grayscale are simultaneously carried out, so that 2m−(2m−n−1) patterns of the display grayscale can be acquired.
In other words, this technique can realize the multi-grayscale display which surpasses the ability of the D/A conversion circuit, so that the increase of the areas occupied with the D/A conversion circuit and the drive circuit can be avoided and the TFT liquid crystal panel can be further downsized.
However, the first and second objectives are not fully achieved with the aforementioned techniques, when the TFT liquid crystal panel is used as the display means of the portable device.
First of all, a rigorous examination of the power consumption of the TFT liquid crystal panel has proved that the D/A conversion circuit is the biggest consumer of electricity among the drive circuits. More specifically, the D/A conversion circuit generates an intermediate voltage from an externally supplied power supply voltage, and supplies the intermediate voltage to a source electrode of the TFT. Thus on the occasion of generating the intermediate voltage (i.e. a display voltage), a lot of electricity is consumed.
Concerning this, the technique (2) is arranged such that the number of bits is reduced to avoid the complication of the D/A conversion circuit. On this account, it is possible to supply a power supply voltage, including the voltage for the power consumption of the D/A conversion circuit, from the external power source, so that the increase of the power consumption can be restrained. However, in this arrangement, corresponding to the time ratio grayscale display, a frequency supplied from the D/A conversion circuit is multiplied by a factor of (m−n) and in proportion to this increase of the frequency, the power consumption of wire capacitance increases.
In the meantime, when a binary output buffer circuit is adopted instead of the D/A conversion circuit as in the technique (1), the increase of the power consumption due to the D/A conversion circuit can be avoided. However, in this case, also a frequency supplied from the buffer is multiplied by a factor of m (bits) so that the power consumption of wire capacitance increases.
As described above, the source electrode of the TFT in the liquid crystal panel has a load-carrying capacitance C, so that in the case of carrying out the time ratio grayscale display, the increase of the power consumption in accordance with this load-carrying capacitance has to be taken into consideration. The increase of the frequency in accordance with the time ratio grayscale gives rise to the increase of the power consumption, and this hinders the reduction of the power consumption.
Incidentally, the larger the dimensions of the panel is, the more the influence of the load-carrying capacitance C of the source electrode is prominent. This load-carrying capacitance C and a resistance R of the source electrode determine a time constant CR of the rise (drop) of the output waveform of the source driver. Thus, on the occasion of carrying out the time ratio grayscale display, the output frequencies of the source driver and the gate driver are multiplied by a factor of the number of bits (generally 6–8 bits), and when the dimensions of the panel further increase, the speed of the rise (drop) of the output waveform of each of the drivers becomes slower than the speed necessary for the time ratio grayscale. Resolving this problem is the third objective of the present invention.
To reduce the load-carrying capacitance C of the source electrode, there are methods such as changing the arrangement of the liquid crystal panel and reducing the relative permittivity of an interlayer insulating film included in the TFT. However, no matter which method is adopted, the arrangement of the liquid crystal panel has to be significantly changed, and hence the increase of the costs, the alteration of the manufacturing process, etc. are inevitable so that adopting the aforementioned methods is considered to be unrealistic.
Consequently, both the techniques (1) and (2) cannot achieve the objectives (1) and (3) adequately.
The technique (2) adopts a D/A conversion circuit with an ability of voltage grayscale of n bits so as to realize the multi-grayscale display surpassing the ability of the D/A conversion circuit. However, among the drive circuits in the TFT liquid crystal panel, a source driver for inputting image data has to have an ability corresponding to the above-mentioned ability of the voltage grayscale of n bits. Moreover, even though the D/A conversion circuit can be arranged without complicacy, the increase of the area occupied with the D/A conversion circuit cannot be avoided adequately. Thus the area occupied with the source driver cannot be reduced and hence the second objective cannot be achieved sufficiently by this technique.
Apart from the liquid crystal panel, recently an organic EL display using an organic EL device has been considered as a promising candidate for the display means of the portable device. In this organic EL display, however, the problems related to the D/A conversion circuit and the source driver also occur, as in the case of the liquid crystal panel. Thus, after all the first, second, and third objectives have to be achieved adequately too, when the organic EL device is adopted as the display means of the portable device.